There have been provided a variety of amorphous silicon photosensitive members. And such amorphous silicon photosensitive members have been evaluated as being high in the surface hardness, exhibiting a high sensitivity against a long wavelength light such as a visible light (from 400 to 700 nm) and a semiconductor laser beam (from 770 nm to 800 nm) and its surface being maintained uniform even upon repeated use for a long period of time. In view of this, they have been desirably used as an electrophotographic photosensitive body, for example, in high-speed electrophotographic copying apparatus or laser beam printer.
The image-forming method in any of these cases is carried out, for example, in the way as shown in FIG. 3. FIG. 3 is a schematic explanatory view illustrating a typical embodiment for carrying out the image-forming method in the conventional electrophotographic copying apparatus. As shown in FIG. 3, near a cylindrical photo-sensitive member 301 to be maintained at a temperature of 42.degree. to 45.degree. C. which rotates in the arrow direction, there are provided a main corona charger 302, an electrostatic latent image-forming mechanism 303, a development mechanism 304, a transfer sheet feeding mechanism 305, a transfer charger 306(a), a separating charger 306(b), a cleaning mechanism 307, a transfer sheet conveying mechanism 308 and a charge-removing lamp 309.
The cylindrical photosensitive member 301 is uniformly charged by the corona charger 302 to which a high voltage of, for example, +6 to +8 KV is impressed. Then, an original 312 to be copied is irradiated with a light from a light source 310 such as a halogen lamp of 50 to 80 V and 200 to 400 W through a contact glass plate 311 and the resulting light as reflected is projected through mirrors 313, 314 and 315, a lens system 317 containing a filter 318 and a mirror 316 onto the surface of the photosensitive member 301 to form an electrostatic latent image corresponding to the original 312. This electrostatic image is developed with negative toner supplied by the development mechanism 304 to provide a toner image. A transfer sheet P is supplied through the transfer sheet feeding mechanism 305 comprising a transfer sheet guide 319 and a pair of feed timing rollers 322 so that the transfer sheet P is brought into contact with the surface of the cylindrical photosensitive member 301, and corona charging is effected with the positive polarity different to that of the toner from the rear of the transfer sheet P by the transfer charger 306(a) to which a high voltage of +7 to +8 KV is applied in order to transfer the negative toner image onto the transfer sheet P. The transfer sheet P having the toner image transferred thereon is electrostatically removed from the cylindrical photosensitive member 301 by the charge-removing action of the separating corona charger 306(b) where a high AC voltage of 12 to 14 KV.sub.p-p is impressed with 300 to 600 Hz and is then conveyed by the transfer sheet conveying mechanism 308 to a fixing zone (not shown).
The residual toner on the surface of the cylindrical photosensitive member 301 is removed by a cleaning blade 321 when arrived at the cleaning mechanism 307 and the removed toner is discharged by way of a waste toner-discharging means (not shown). Thereafter, the thus cleaned cylindrical photosensitive member 301 is entirely exposed to light by the charge-removing lamp 309 to erase the residual charge and is recycled.
The amorphous silicon photosensitive member to be used in the above image-forming method has such advantages as above mentioned, that is, it has a high sensitivity also against not only a visible light (from 400 to 700 nm) but also a long wavelength light (sensitivity peak near 680 nm and sensitivity region of 400 to 800 nm), but in turn, has a disadvantage that, there sometimes occurs a problem that reproduction of, for example, a red colored seal in a original is not sufficient when it is used in the image-forming method in an analog electrophotographic copying apparatus having a halogen lamp as the image-forming light source mainly because of defects in the matching of the light source and the photosensitive member. In order to eliminate this disadvantage and to ensure the red reproduction, there has been proposed use of a long wave length light (IR) cutoff filter such as soda glass.
However, there is a problem in this case that the total quantity of light from the light source to be irradiated for the formation of an electrostatic latent image will be somewhat reduced because of using such cutoff filter and in order to supplement the deficient quantity of light, it is necessitated to heighten the wattage of the light source. This situation is apparent particularly in the case of a high-speed electrophotographic copying apparatus in which the image-making process is quickly carried out and the period of image exposure is much shorter in comparison with the case of a normal-speed electrophotographic copying apparatus.
By the way, in the recent office automation market, there is a increased demand for provision of a high-speed electrophotographic copying apparatus of low electricity consumption for which a power source of 100 V and 15 A can be used in view of remarkable increase in the amount of papers to be copied, reduction of expenses for the copying, etc.
On the other hand, there is a problem for attaining the above demand that electric power is remarkably consumed for the main driving motor, light source, fixing heater, sorter, etc. in the high speed copying system.
Now, as for the amorphous silicon photosensitive member, it is evaluated as being the most suitable for use in a super-high speed heavy duty copying apparatus because of excellence in stability and abrasion resistance in addition to the foregoing advantages.
However, it is extremely difficult to raise the electric power to be applied so as to increase the quantity of light to be irradiated in order to ensure the red reproduction in the high speed electrophotographic copying apparatus in which such amorphous silicon photosensitive member being installed, because of the foregoing demand for low electricity consumption and an increase in additional electric power distribution for attached equipments.
Therefore, the maximum copying speed for the high speed electrophotographic copying apparatus under which acceptable reproduction of a red-colored seal in an original is ensured is of the level of 440 mm/sec. for the image-forming process speed (in other words, 70 copies per minute for a A-4 size original. And in the case where it is intended to provide such an electrophotographic copying apparatus which is of the copying speed exceeding the above level, the matter of the red reproduction is to be somewhat sacrificed regardless of dissatisfactions of users.
Independently from what above mentioned, in recent years, there has been a demand from users for electrophotographic copying apparatus to be able to reproduce multicolored images comprising red, blue, etc. other than black. In such copying apparatus, a plurality of development mechanisms are provided with a photosensitive member so as to make multicolored images.
However, in the case of using an amorphous silicon photosensitive member in such color-copying apparatus, there sometimes occur charge decay in dark. In order to avoid this problem, it is necessary to make both the charge and the exposure for forming an electrostatic latent image remarkably large for the development mechanism positioned apart from the corona charger in comparison with those for the development mechanism positioned near the corona charge.
On the other hand, the electrophotographic copying apparatus is used with an electric power source of 100 V and 15 A and because of this, there is a limit for the total electric power. In view of this, for an electro-photographic copying apparatus capable of making multicolored images in which an amorphous silicon photosensitive member is used, it is extremely difficult to ensure the red reproduction.